The relation between sintering mechanism and electrical conductivity of zinc oxide, a typical representative of oxide semi-conductor, was studied from a view point of rate-process. On the degree of sintering P and velocity equation of sintering, expressions (1) and (2) were examined.P=ρa-ρ0/ρt-ρ0…(1)dP/dt=K(100-P/100)n…(2)Where symbols denote:-ρa=bulk densities of specimens after sintering.ρ0=bulk densities of specimens before sinteringρt=true density of specimensK=velocity constantn=order of reactiont=duration of sinteringNow it was established that the expressions (6) and (7) are valid between n and K as functions of φ and ψ which are indentically equal to values of log (100-P) when the reaction velocity (dP/dt) becomes unity or 100, respectively. (Fig. 4)n=2/(ψ-φ)…(6)log K=n(2-φ)…(7)For simple one component system such as metallic oxides, to which zinc oxide used in the experiment also belongs, or metal and glass powders., order the of reaction n will be constant over the range of sintering temperature. In the case of zinc oxide, n was found to be 5.7 in a range of 800-1250°C, while the value of φ was expressed as a certain function of temperature. In two or more components system, n is not a constant but a function of temperature. For instance, it was found with powdered clayey raw material for electric insulating porcelain that value of n decreases lineally with temperature.As expressed in equation (8), the velocity constant K, has a physical meaning of reciprocal of n′th power of q0, “degree of not-sintering” which is equal to the percentage of remainder of sintering.K=(q0)-nwhere, q0=(100-P0)/100…(8)Result of experiments with zinc oxide show their activation Energy E to be 74.6kcal/mol. in case of firing at 800-1250°C in common air atomosphere and its sintering process can be expressed as followsdP/dt=K(100-P/100)5, 7log10K=14.9-1.6×104/T}………………………………(9)The electrical resistivity of zinc oxide is predominantly determined by diffusion of zinc atom librated by dissociation of ZnO in oxide lattice. Electrical resistivity, ρ decreases with increase of sintering time as expressed by the equation, ρ=At-B, obtained experimentally, where A and B are constants depending on sintering temperature. But the resistivity attains finally at a saturation value after some time of firing, necessary duration of which is also some function of temperature. (Fig. 7) The logarithm of saturation value is found to be a linear function of the ratio, (γ) of sintering and melting temperature as shown in equation (11).log ρsat.=15.4-20.3γ……………………………………(11)Then the relation between velocity constant K and the saturated value of electrical conductivity, σsat. is expressed asσsat.=4.47×10-6[K]1.15…………………………………(12)In this equation, it is seen that σsat. is nearly proportional to K. This means that amounts of intersticial zinc atom diffused in oxide